1. Field of the Invention
The present invention relates to a semiconductor device used for power control, and particularly to a nitride semiconductor device such as nitride semiconductor field effect transistor or Schottky barrier diode.
2. Related Background Art
A nitride semiconductor device where gallium nitride (GaN) is used as part of materials has a larger band gap and a higher critical field as compared with a semiconductor device which does not contain gallium nitride and uses silicon (Si) as main material so that the device having a small size and a high breakdown voltage can be easily achieved. Thus, it is possible to achieve a semiconductor device having low on resistance and low loss also as a semiconductor device for power control.
In particular, an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure field effect transistor (HFET) can be expected for preferable characteristics because of its simple device structure.
Since it is difficult to manufacture a substrate made of single gallium nitride in the aluminum gallium nitride/gallium nitride heterostructure, the substrate is generally formed by crystal growth on a substrate such as a sapphire substrate or a silicon carbide (SiC) substrate. Since the material such as sapphire or silicon carbide has a lattic constant relatively similar to the aluminum gallium nitride/gallium nitride heterostructure, a gallium nitride crystal film having a thickness of about several μm can be grown on the sapphire substrate or silicon carbide substrate without generation of any crack.
However, there is a problem that, since the sapphire substrate has large thermal resistance, radiation from a device formed on the substrate is difficult, and further there is a problem that the substrate having a large diameter is difficult to manufacture so that even the substrate having a small diameter on the order of 2 to 3 inches is expensive.
On the other hand, no problem occurs on the radiation from a device formed on the substrate because the silicon carbide substrate has small thermal resistance, but there is a problem that the substrate having a large diameter is difficult to manufacture like the sapphire substrate and even the substrate having a small diameter is expensive.
Totally judging, it is a probable measure that the substrate having a large diameter can be manufacture at low cost and the aluminum gallium nitride/gallium nitride heterostructure is formed using a silicon substrate having relatively small thermal resistance to manufacture the device.
However, since the lattic constant of silicon is greatly different from that of the aluminum gallium nitride/gallium nitride heterostructure, a crack due to distortion easily occurs and only a gallium nitride layer having a thickness of about 1 μm to 2 μm can be crystal-grown in order to avoid generation of crack.
Since the upper limit of the breakdown voltage of the gallium nitride layer contained device on the silicon substrate is determined depending on the thickness of the gallium nitride layer, the breakdown voltage of at most about 200 V can be secured at present, and a nitride semiconductor device having a high breakdown voltage of 600V, 1200 V, or more is difficult to achieve.
It has been proposed and known a method of forming a semiconductor layer capable of restricting warpage of a substrate when a semiconductor layer having a thermal expansion coefficient different from the substrate is formed. See Japanese Patent Application Laid-Open No. 2003-17409 Publication, for example. However, in the method disclosed in Japanese Patent Application Laid-Open No. 2003-17409 Publication, a warpage prevention layer is formed on the semiconductor layer so that a structural limitation is imposed on the semiconductor device.
Therefore, when a semiconductor layer having greatly different lattic constant or thermal expansion coefficient is formed on the semiconductor substrate having a large diameter, a problem that crack or warpage easily occurs on the substrate cannot still solved basically.